In general, the structure of optical fiber consists of two coating layers of resin composition on the surface of a bare optical fiber.
Usually, a flexible coating material, called a soft material, is used to form the first coating layer and a high rigid coating material, called a hard material, is used to form the second coating layer.
The ultraviolet curable resin (hereafter, the UV resin), of which the principal ingredient is the oligomer of the urethane acrylate or/and the epoxy acrylate, is adopted as the resin composition of the coating layer from the viewpoint of the curing rate.
An optical fiber is manufactured by coating such an UV resin liquid on the surface of a bare optical fiber, and curing it by irradiating ultraviolet rays continuously during drawing.
In this optical fiber, the degree of stickiness on the surface of the outermost coating layer is called a tack of surface. This tack of surface is an important factor that influences the ease of handling and manufacturing in the post-processing, and it is necessary to suppress it to be low.
In a usual optical fiber, the second coating layer becomes the outermost coating layer.
An optical fiber wound round the bobbin after drawing is rewound up to the shipment bobbin, and is cut by the predetermined length. This process is called rewind process.
Next, a wound optical fiber is passed to the inspection process, and the transmission loss is measured. OTDR is usually used to measure this transmission loss in which, an optical pulse is launched into the optical fiber and the backscattered light which is reflected from each point in the longitudinal direction of the optical fiber is measured on the axis of time.
As a measurement result by OTDR, attenuation curve of optical power, of which the vertical axis shows the optical power level and the horizontal axis shows time (length), is obtained as shown in FIG. 5. The transmission loss over the total length of the optical fiber is measured from this attenuation curve.
In this attenuation curve, the loss variation like the step is called discontinuity (refer to FIG. 5).
Such a discontinuity mostly disappears by rewinding it several-times. However, when every time the discontinuity is generated, the OTDR measurement has to be done again and again which increases the workload of the rewind process and it becomes a problem.
Especially, when the tack of the surface of the optical fiber is high, the discontinuity originating from it occurs, and the rate of occurrence of the discontinuity becomes high.
The method of manufacturing of the optical fiber that reduces the tack of surface is proposed in “JP patent No. 2614949”, for instance, in which the curing is done by the ultraviolet rays irradiation, under the atmosphere of 1% or less of oxygen concentration.
The patent shown above is related to the prevention technique of the increase of the tack, which is occurred by that the radical initiator cleaved by the UV rays irradiation is trapped by the oxygen and the resin of the optical fiber coating surface is insufficient curing condition.
Moreover, decreasing of the rate of occurrence of the discontinuity is tried by lowering the coefficient of kinetic friction on the surface of the outermost coating layer of optical fiber (hereafter, the coefficient of kinetic friction of the optical fiber).
When the coefficient of kinetic friction of the optical fiber is high, it will cause the generation of discontinuity, because the loss increases by microbending, the taking up and paying out to the bobbin become difficult, and/or the damage in coating layer of optical fiber occurs, in extreme case, by generation of kinetic friction force between optical fibers and also between an optical fiber and a bobbin.
JIS K7125 provides the general measurement method of a coefficient of kinetic friction by using the film formed with the same quality as the measured material. However, when the coefficient of kinetic friction of optical fiber is measured by the method of the above-mentioned JIS, the variations in the measurement results are extremely large, and it is difficult to obtain a reliable value.
The method of measuring the coefficient of kinetic friction in the colored optical fiber, overcoming this problem, is indicated in “Japan Laid-Open Patent Application (JP-A;KOUKAI) No. Hey11-194071”. (The colored optical fiber is the one in which the UV resin that includes pigment was covered on the above-mentioned optical fiber).
In this method, one optical fiber is fixed on a horizontal fixed table, and the other is fixed on the bottom of a sled substantially in parallel with the former one. The sled is loaded on the fixed table so that each optical fiber cross right-angled, and they contact. The kinetic frictional force caused by the movement of the sled is measured.